The present invention relates to an optical instrument for measuring displacement and more particularly to such an instrument which utilizes diffraction of monochromatic light from a grating which is movable relative to a light source or sensing head.
A number of systems have been proposed heretofore for measuring relative displacement utilizing diffraction of light from an optical grating. Examples of such prior art systems may be seen in the Pettigrew U.S. Pat. No. 4,776,701; the Kanayama et al. U.S. Pat. No. 4,815,850; the Taniguchi et al. U.S. Pat. No. 4,676,645; and the Wijntjes et al. U.S. Pat. No. 5,098,190. A commercially available system of this type is sold by Optra, Inc. of Beverly, Mass. under its trademark "Nanoscale". U.S. Pat. No. 5,098,190 is believed related to the "Nanoscale" product. Each of these prior art systems, however, involves magnification or separation of the different diffraction orders obtained back from the diffraction grating which are then brought back together and interfered. Typically, fairly large numbers of optical components are required and the several components must be accurately spaced and aligned in order for the instruments to perform in accordance with their respective designs.
In many high-tech industries products are manufactured and/or operated under "clean room" conditions. For example, the progress made in miniaturizing mechanical systems, or in obtaining ever increasing densities in integrated circuits, has been due in part to the quality of such "clean rooms". On the other hand, clean room requirements are costly to implement, can complicate manufacturing procedures, and can increase the cost of test equipment and components which might be needed in the clean room. It is desirable to have a method and apparatus which permits controlling the movement of components within a closed space from a point outside of the closed space.
Among the several objects of the present invention may be noted the provision of apparatus for detecting relative displacement which employs a minimum of components; the provision of such a system in which tolerance of spacing and alignment of system components is relatively great; the provision of such apparatus which is easily manufactured; the provision of such apparatus in which sensing components can be implemented using integrated circuit techniques; the provision of such apparatus which can provide measurement with high accuracy; and the provision of such apparatus which is highly reliable and which is of relatively simple and inexpensive construction. Other objects and features are in part apparent and in part pointed out hereinafter.